The panels forming roof panels have to fulfill some requirements in the best possible way, for example must be thin, light, stiff, and inexpensive; must exhibit good waterproofing and sound absorption properties; and must be produced with materials having a low emission of substances that are volatile or potentially harmful to health.
Typically roof panels are composed of composite panels made of polyurethane foams and synthetic or mineral fibers such as glass fibers.
Polyurethane foams however have a relatively low stiffness and, when used as cores for composite panels, require a relatively high thickness, adversely affecting weight and cost of the finished panel.
At the same time mineral or glass fibers, which are used as a reinforcing layer, are a material that is hard to handle, and are potentially hazardous to health, above all when provided in high amounts.
Moreover, the adhesion of the several layers of the panel is generally achieved by using adhesives, which need solvents that can cause over time gas emissions inside the car passenger compartment, increasing health hazards.
Several attempts have been made to obtain high stiffness and mechanical strength for the panels in combination with weights as small as possible. However such attempts have not led to satisfactory solutions, that is, improving and maximizing the ratio of mechanical strength or stiffness to the weight of a composite panel.
U.S. Pat. No. 6,706,370 describes a panel for roof panels of cars that is composed of a core of foamed polyurethane covered on two faces by layers of glass fibers impregnated with resin. The provision of glass-reinforced plastic provides an excellent mechanical strength, but a high weight above all due to glass fibers.
EP 1878568 describes a composite panel with a core composed of two layers a foamed and semi-rigid one and a mat of plastic fibers. The foamed layer is made of foamed polyurethane and the reinforcing layers superimposed on the faces of the core layer comprise glass fibers or other natural or mineral fibers. Even in this case therefore obviously the drawbacks of known panels are present, by which given a certain mechanical strength or stiffness a still relatively high weight is provided.
US 2004/0234744 describes a composite panel having a foamed core made of foamed polyurethane. On the contrary the reinforcing layer or layers are made of mineral fibers, such as for example basalt fibers impregnated with a thermoplastic binder, therefore given a certain stiffness or strength, mineral fibers still lead to a relatively high weight of the panel for types of panels described in the above mentioned documents.
EP 1844927 describes a fiber-reinforced sheet to be used in the automotive field whose core however is not made of foamed material. The reinforcing fibers are of the synthetic, metal, mineral or natural type and therefore they have a certain weight, therefore even in this case the stiffness/weight or mechanical strength/weight ratio is not satisfactory.
WO2006/060881 describes a film made of thermoplastic material reinforced with basalt fibers. This film does not provide a core of foamed material and mineral fibers, particularly basalt fibers are used as reinforcing fibers and therefore it follows the same path followed by the solutions described in the above mentioned documents.